Consider a power control system for light emitting diodes (LED) as shown in FIG. 1 (which depicts an embodiment to be discussed below). The purpose of this system is to provide controlled current for operation of the N LED strings STR denoted as 10 through 12. To this end, a multiplicity of N current sinks I1 through In denoted as 1, 2, and 3 are used to control the current through the LED strings. These current sinks may have different values, or may be operating at different times, without affecting the considerations being discussed below. A series combination of a current sink 1, a switch 4, and an LED string 10, for example, is denoted as a channel. The voltage source V1 is optimally chosen to have a value just large enough that all of the current sinks operate correctly. If the channel voltages VCH1, VCH2, through VCHn are of sufficient magnitude, the current sinks are able to control the current flowing through the associated LED string. Practical realization of the power control system is usually done by an integrated circuit as known in the state of the art, associated with a few external components.
For normal operation, all the LED strings STR1 through STRn will have similar voltage drops for the amount of sink current flowing through the strings. In this case, power dissipation in the current sinks caused by the channel voltages VCH1 through VCHn will be relatively small, giving efficient production of light output by the LEDs without wasting input power.
During normal operation, a voltage detector circuit 14 is used to determine the channel which has the minimum value of VCH, and uses that voltage to provide minimum voltage feedback to control the power source 13 for all the LEDs. In this way, the channel with the lowest value of voltage across its current sink is provided just sufficient voltage so that the current sink works correctly. All other channels have higher voltages for VCH, so their current sinks also work correctly. Normal statistical variations in the operating voltage drops of the LEDs will cause the channel voltages VCH to vary among the channels, with the lowest channel voltage controlling the power source 13 to generate an optimum voltage V1.
Operation of the LED strings begins with the channel enable signals 17 being turned on, so that a memory device in the control memory 16 associated with each LED string 10 to 12 is turned on, thereby closing the switches SW1 through SWn. When these switches are closed, current from the voltage source 13 can flow through the LED strings 10 to 12 to the current control sink circuits 1 to 3.
One objective of this disclosure is to discuss a means for performing the voltage detection in block 14 so as to find and disconnect failed LED strings, thereby preventing damage to the integrated circuit system. A further object is to provide a means for improving the power efficiency of the LED system by minimizing power dissipated, thereby reducing the total power consumed in production of a given amount of light output from the LEDs. If the integrated circuit system is dissipating excessive power as heat, this power does not contribute to the light output of the LEDs, but it will reduce the operating lifetime of a battery power source.
In an adverse operating condition, one or more of the LED strings may have one or more failed LEDs, said LED having either a larger or a smaller voltage drop than normal. If this causes the voltage across one or more channel current sinks to be too large, the power lost in the current sinks will cause excessive device heating. In this case, some means must be provided for determining which of the LED strings has the failed device and removing the string from usage.
The voltage detector 14 has several sets of outputs. The signals CHH tell which of the channel voltages VCH is the highest, signals CHL tell which of the channel voltages VCH is the lowest, and signal SNO tells whether the fault is likely to be due to an excessively high or low voltage. These signals go to a fault logic block 21, where logical combinations of the above signals are used to determine which LED channel is faulty so it can be turned off. The fault logic block 21 provides a set of outputs 15 denoted ERS, typically on separate wires, which can denote the presence of a failed LED string and assist in turning it off. These outputs are used to connect to a control memory block 16, which receives the channel enable signals 17 denoted CHEN together with a trigger signal TR on 20 and generates the control signals CHON on 18 to the switches 4 through 6 in each channel. An active CHEN signal initially turns on the current sink for an LED channel, and an active TR signal indicates that a fault is present and the power dissipation needs to be reduced. When the CHON signal is active, the corresponding LED channel is allowed to operate. If the CHON signal is not active, then the current sink for the LED channel is turned off, and the channel voltage VCH is no longer used to help control the voltage V1 of the power source 13. The control memory block typically contains a memory device for each channel, so that once a channel is recognized as having a failure, that channel can be turned off and the presence of the failure will be remembered.
Consider the case when an LED string has a device which has a large operating voltage drop, or is an open circuit, causing the corresponding channel voltage VCH to drop towards zero. The minimum voltage feedback value to the power source 13 will correspondingly fall to zero, causing the power source 13 to increase its output V1 until the minimum channel voltage is brought back to its desired value. As a result, the value of VCH for all other channels will be increased, causing the power dissipation in the current sinks of all other channels to increase. This can lead to excessive power dissipation in the overall system used to create the current sinks, damaging the integrated circuit. In the case where an open device is present, the voltage source 13 may increase its output V1 until some device in the system suffers breakdown and damage due to excessive applied voltage. This can result in catastrophic failure of the entire LED illumination system. Usually a separate, independent circuit is used to limit the voltage excursion of the voltage source 13 under these conditions to prevent catastrophic failure.
Therefore, one objective of the voltage detector 14 is to be able to determine if a large voltage drop string STR is present, and isolate it from the operation of the remainder of the system to prevent power loss, overheating, or catastrophic damage.
Now consider the case where an LED string has one or more devices which have less voltage drop than normal or even are shorted out and having no voltage drop. If a sufficient number of these devices are present in a particular string, then the corresponding current sink (1, for example) would have excessive power dissipation. If several LEDs have failed, this power dissipation can become sufficient to endanger the continued operation of the integrated circuit system. In this case the voltage detector 14 would cause the fault logic block outputs 15 to indicate which of the channels has excessive voltage VCH present at its current sink 1. The information is then used by the control memory block 16 to remember which string has the fault, and the control memory sends a signal on one of the wires 18 to turn off the switch which is associated with the failed string. As an example, if some of the LEDs in string STR1 (item 10) have less voltage drop than normal, the voltage VCH1 may cause excessive power dissipation. In this case, the voltage detector 14 would send a signal on one of the wires 15 to cause the memory device in the control memory 16 associated with switch SW1 (item 4) to turn off. The string STR1 would then not draw power or cause excessive power dissipation in current sink I1 (item 1).
Therefore another objective of the voltage detector 14 is to be able to determine if an LED string STR has less voltage drop than the remaining strings, and isolate it from the operation of the remainder of the system to prevent power loss, overheating, or catastrophic damage.
The question of whether a fault condition exists is determined by other circuitry not shown here, which may typically operate to declare a fault condition if the integrated circuit temperature becomes excessive, if the voltage VCH on any individual wire becomes more than a predetermined value, or if the power source 13 has an output voltage V1 greater than a safe value. Other criteria for presence of a fault may also be used. The purpose of the circuit discussed here is to determine without ambiguity which of the LED channels has the fault. If a fault is judged to be present, the trigger wire TR becomes active to cause the error detection and control circuitry to turn off the defective LED channel.
Determination of which channel has the fault can be done by a voltage detector with a block diagram as shown in FIG. 2, in conjunction with the fault logic which will be shown later in FIG. 7. This circuit works by determining the maximum, minimum, and average values of the active channel's VCH inputs taken as a group, and performing computations with those values to determine which of the inputs is responsible for the error. The output signals from this voltage detector are then used by the fault logic 21 and the control memory 16 to take action to turn off the faulty channel. The fault logic and control memory will be detailed separately later.